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The level of the surface of the ocean, based on the average level between high and low tides, which is used as a datum when calculating the height of features on land or depth in the ocean. It is used as a standard base for measuring heights and depths on the Earth, and it is the baseline for plotting the global hypsometric curve, and for calculating the heights of mountains and the depth of ocean trenches. Atmospheric pressure readings are usually expressed as sea level equivalents in order to eliminate the effects of altitude.